When providing voice-over-IP services using an IP-Centrex-style PBX, one possible option is to utilize a remote IP-Centrex server and to have no such server on the local premises of the customer, as is shown in FIG. 1. However, all telephony availability is dependent on Internet connectivity to that remote server. This means that even intra-company telephony services, such as voicemail and intra-company-to-intra-company calling, can be lost when Internet connectivity is lost. This Internet point of failure is not overcome by using redundant remote servers for improved sever availability because they too need to be reached using the Internet.
As an alternative to a solution that uses only remote servers, it is possible to use a set of redundant servers on the customer's premises such that if there is a problem with one server, another server can be used locally as a back-up. However, this solution increases the cost of operation under normal operating conditions (i.e., when the main server is not experiencing any problems).
Accordingly, it would be beneficial to be able to have intra-company telephony services, even in the presence of a failure of an Internet connection, without having the associated cost of a local, redundant back-up server for use during failures of the main server.